The present invention generally relates to a device, a system, and a method for controlling the voltage on a wind farm collector bus during and after a low-voltage or grid fault occurrence on a utility power grid, to prevent the wind turbine generators from tripping and disconnecting from the power grid.
There has been an increasing interest in non-fossil fuel energy sources as a consequence of the escalating cost and political ramifications associated with a reliance on foreign oil. Wind turbine generators (“WTGs”) are one such energy source. WTGs offer the further benefit of providing an environmentally-friendly, non-polluting energy source.
WTGs are controlled current sources that normally deliver current at approximately unity power factor. WTGs rely on wind force to rotate a plurality of turbine blades. The turbine blades are coupled to the rotor assembly of a generator. As the turbine blades rotate, a plurality of windings on the rotor assembly induces current to flow in a plurality of windings on a stator. The current can be converted and phased as necessary and the voltage may be stepped-up, e.g., using a transformer, to produce electrical power at a desired voltage.
Although individual, isolated WTGs may be erected to service a small number of customers, typically, WTGs are structured and arranged in groups or “farms” that are electrically coupled to a collector bus to provide power to a utility power grid for a multitude of customers.
For the greatest efficiency and for maximum power output, it is desirable that the WTGs operate within about ±10 percent of their rated, or nominal, operating voltage capacity, i.e., between about 90% and about 110%. However, during operation, if the WTG controller senses that the voltage on the collector bus is below a threshold voltage, e.g., about 70 to 85% of the nominal operating voltage, the controller is designed to disconnect the generator from the grid to protect the WTG generators.
Although it is important to disconnect the WTG from the power grid to protect the WTG from a fault condition on the power grid, when a WTG disconnects from the power grid, it may take several minutes to go through a restart cycle and come back on line. During this restart time, the WTGs are not available as a power source. As a result, a transient voltage sag in the utility power grid, in the range of milliseconds, could cause a complete shutdown of an entire wind farm power source for several minutes.
The Federal Energy Regulatory Commission (“FERC”) has proposed minimum requirements for wind plant response to low-voltage conditions on a utility power grid. FIG. 1, shows the FERC's proposed minimum required wind plant response curve with respect to time.
Prior to a grid fault, e.g., at time minus one second (t−1), each WTG should be operating at ±10% of nominal capacity (Point A). When a low-voltage grid fault occurs (at t0), the voltage at the high side of the wind farm transformer can decrease instantaneously by as much as 90 percent, e.g., from 100 percent to about 10 percent of its nominal operating voltage (Point B TO Point C). When such a voltage drop occurs on the collector bus, the FERC mandates that the WTGs in the wind farm be able to ride-through the low-voltage condition for at least 625 milliseconds (POINT D). After about 625 milliseconds, power from the WTGs should again be deliverable, i.e., brought on line, to the utility power grid (Point E) and power should be fully restored from the WTGs within three (3) seconds of the occurrence of the fault at t0. The voltage at Point E will be at least 90% of the nominal operating voltage capacity of the WTGs.
Thus, it would be desirable to provide a device, a system, and a method that complies with the low-voltage fault ride-through requirements established for WTGs by the FERC minimum. It would further be desirable to provide a device, a system, and a method for providing a low-voltage fault ride-through that includes a control system that prevents the WTGs from disconnecting from the utility power grid during the ride-through period.